The present invention pertains to improvements in the field of thermosetting polymer compositions. More particularly, the invention relates to a curable vinyl ester resin composition which is capable of undergoing B-staging at room temperature and which is suitable for impregnating a fibrous material.
In the conventional production of a resin pre-impregnated fibrous material, the fibrous material is impregnated with a curable resin dissolved in a solvent and the resin is partially cured to a B-stage by the application of heat followed by a solvent recuperation. A B-stage is a temporary phase which occurs after the resin has undergone an increase in viscosity without solidification and during which the increased viscosity remains stable. The pre-impregnated fibrous material is then stored or shipped to an end-user who completes the cure with heat and pressure in a molding operation. Such a process is costly since it requires the application of heat during the partial curing and a subsequent solvent recuperation.
It is known that a Group II metal oxide or hydroxide, such as magnesium oxide, is capable B-staging thermosetting vinyl ester resins at room temperature only if the resins have been chemically modified to contain free carboxylic acid groups, such as described in U.S. Pat. No. 3,548,030, or modified to contain an increased proportion of ester groups, such as described in U.S. Pat. No. 4,197,390. The processes described in these two patents are also costly since they require the preparation of a special vinyl ester resin.
It is therefore an object of the present invention to overcome the above drawbacks and to provide a curable vinyl ester resin composition wherein the vinyl ester resin is a conventional unmodified resin which can nevertheless undergo B-staging at room temperature.
In accordance with the present invention, there is thus provided a curable vinyl ester resin composition capable of undergoing B-staging at room temperature, comprising:
a) an unmodified vinyl ester resin;
b) a reactive diluent consisting of an ethylenically unsaturated monomer;
c) a free-radical curing agent;
d) a B-staging agent comprising a Group II metal oxide or hydroxide, or a mixture thereof; and
e) a thickening agent comprising an ester, a ketone, an aldehyde or a mixture thereof;
wherein the thickening agent coacts with the B-staging agent to cause B-staging of the vinyl ester resin at room temperature.
Applicant has found quite unexpectedly that organic compounds containing one or two ester or carbonyl groups such as found in most readily available esters, ketones and aldehydes synergistically coact with Group II metal oxides or hydroxides to cause B-staging of a conventional unmodified vinyl ester resin at room temperature. If the ester, ketone or aldehyde is used without the Group II metal oxide or hydroxide, it does not cause B-staging of the resin. On the other hand, if the Group II metal oxide or hydroxide is used alone, it is known that such a metal oxide or hydroxide is not capable of B-staging an unmodified vinyl ester resin at room temperature.
Examples of suitable esters which may be used in accordance with the invention include ethyl acetate, octyl acetate, ethyl butyrate, diisobutyl phthalate and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate. Diisopropyl phthalate is preferred. It is also possible to use, as thickening agent, a ketone such as acetone, 2-butanone and methyl isobutyl ketone, and/or an aldehyde such as acetic aldehyde, propionic aldehyde and 3-methyl butanal. Methyl isobutyl ketone is preferred. A particularly preferred thickening agent comprises a mixture of diisobutyl phthalate and methyl isobutyl ketone, or a mixture of diisobutyl phthalate, methyl isobutyl ketone and 2,2,4-trimethyl pentanediol diisobutyrate. The thickening agent is generally present in the resin composition of the invention in an amount of about 0.5 to about 3 weight %, preferably about 1 weight %, based on the total weight of the composition.
The B-staging agent is advantageously a Group II metal oxide. Magnesium oxide is preferred. The B-staging agent is generally present in the resin composition of the invention in an amount of about 1 to about 5 weight %, preferably about 2 weight %, based on the total weight of the composition.
The vinyl ester resin is generally used in an amount of about 54 to about 58 weight %, preferably about 58 weight %, based on the total weight of the composition. A particularly preferred vinyl ester resin is an epoxy novolac-based vinyl ester resin.
The vinyl ester resin composition according to the invention further includes a reactive diluent consisting of an ethylenically unsaturated monomer which participates to the cross-linking of the vinyl ester resin during subsequent curing thereof. Examples of suitable reactive diluents which may be used include styrene, vinyl toluene, divinylbenzene as well as the methyl, ethyl, isopropyl and octyl esters of acrylic or methacrylic acid. Styrene is preferred. The reactive diluent is generally present in the resin composition of the invention in an amount of about 35 to about 40 weight %, preferably about 38 weight %, based on the total weight of the composition.
Use is preferably made of an epoxy novolac-based vinyl ester resin diluted in styrene, sold by the Dow Chemical Company under the trade-mark DERAKANE 470-300. Such a diluted resin comprises about 50 to about 70 weight % of the vinyl ester resin and about 30 to about 50 weight % of styrene. It has a viscosity ranging from about 250 to about 550 centipoises.
The vinyl ester resin composition of the invention also includes a free-radical curing agent such as an organic peroxide, to initiate cross-linking of the vinyl ester resin with the ethylenically unsaturated monomer during the subsequent curing of the resin composition. Methyl isobutyl ketone peroxide is preferably used as free-radical curing agent. The curing agent is generally present in the resin composition in an amount of about 0.5 to about 3 weight %, preferably about 1 weight %, based on the total weight of the composition.
A particularly preferred curable vinyl ester resin composition capable of undergoing B-staging at room temperature comprises:
about 58 weight % of an unmodified vinyl ester resin such as an epoxy novolac-based vinyl ester resin;
about 38 weight % of a reactive diluent such as styrene;
about 1 weight % of an organic peroxide such as methyl isobutyl ketone peroxide;
about 2 weight % of magnesium oxide;
about 0.7 weight % of diisobutyl phthalate;
about 0.25 weight % of methyl isobutyl ketone; and
about 0.05 weight % of 2,2,4-trimethyl-1,3-pentanediol diisobutyrate.
The vinyl ester resin composition according to the invention is particularly useful for impregnating a fibrous material such as a woven fabric material. The fibrous material impregnated with the vinyl ester resin composition of the invention has a shelf life of up to 12 months. B-staging is generally reached in about two weeks following mixture of the various components and impregnation of the fibrous material.
The present invention therefore provides, in another aspect thereof, a fibrous material impregnated with a curable vinyl ester resin composition capable of undergoing B-staging at room temperature, wherein the vinyl ester resin composition comprises:
a) an unmodified vinyl ester resin;
b) a reactive diluent consisting of an ethylenically unsaturated monomer;
c) a free-radical curing agent;
d) a B-staging agent comprising a Group II metal oxide or hydroxide, or a mixture thereof; and
e) a thickening agent comprising an ester, a ketone, an aldehyde or a mixture thereof;
the thickening agent coacting with the B-staging agent to cause B-staging of the vinyl ester resin at room temperature.